This invention relates to techniques for oxygenating aerobically decomposable effluent liquors such as raw sewage, treated and untreated wastewater effluents, and other raw and mixed liquors such as those from municipal sewage systems, food processing plants, and industrial plants of various types such as pulp and paper mills, steel mills, and the like. Oxygenation of such raw and mixed aerobically decomposable liquors is for the purpose of biologically oxidizing the impurities in the liquor, as is well known.
Although the invention will be found useful in the treatment of other effluent liquors such as wastewaters produced in pulp and paper mills, food processing plants, and elsewhere as previously indicated, it was made in connection with an attempt to improve well-known activated sludge techniques for treating sewage and will therefore be described in connection with such use.
The purification of sewage and wastewaters by the biological oxidation of their contained impurities has long been known and is most commonly conducted using the activated sludge treatment process. Although variations of the process are known, in general it involves flowing into an aeration tank either the raw sewage or preferably the primary effluent from a primary clarifier together with return activated sludge from a final clarifier, and the introduction of air or oxygen into the mixed liquor to promote the growth of certain aerobic microorganisms which subsequently consume the organic impurities contained in the liquor so as to convert them into harmless, oxidized by-products such as carbon dioxide, ash, water, etc., as is well understood by those skilled in the art. The mixture of liquid and biological floc produced in the aeration tank is separated by settling in a final clarifier into which the mixture flows after aeration, and it is a portion of the settled out biological floc or activated sludge from the final clarifier which is reintroduced to the aeration tank to inoculate the incoming raw sewage upon its introduction into the aeration tank, and which is thus used to speed the growth of the organisms and, of course, the treatment process. The liquid drawn from the final clarifier is substantially pure and may be chlorinated and discharged into a river or reused in an industrial process, with or without its being further processed. This invention is concerned with the aeration stage of such a process.
For an activated sludge treatment process to be successfully conducted, it is necessary that certain environmental conditions of the aerobic organisms be maintained during the aeration stage, among which is a sufficient supply of dissolved oxygen as is necessary to stimulate their growth or multiplication. In addition, it is necessary that the untreated sewage and activated sludge contained in the aeration tank be constantly mixed in a proper manner so that solids which are suspended in the mixed liquor (MLSS) will be retained in suspension during the process and thus more completely exposed to the impure liquor being treated. In this regard, it is known that optimum velocities throughout the aeration tank, including areas adjacent to the tank bottom, should be on the order of from 1.0 to 3.0 feet per second to produce the most desirable degree of mixing. Lower velocities will permit settling of the suspended solids with consequent reduction in their oxidation rate, and higher velocities are not only wasteful of the energy required to produce them but may cause shearing or homogenization with the wastewater of the relatively fragile biologically active floc which is being produced during the aeration process, thus impeding separation of liquid phase products from solid phase products in the final clarifier apparatus to which the mixture is passed following the aeration treatment.
Previous attempts to improve the speed or the efficiency of the aeration process have either intentionally or inherently involved an interdependent relationship between the aeration or oxygenating aspect of the process and the mixing aspect of the process, such that any attempt to improve or control one of these aspects necessarily affects the other. For example, although the mixing rate may be adequate to keep solids in suspension yet avoid homogenization of the floc, aeration tanks fitted with surface aerators may at times require additional amounts of dissolved oxygen in order to adequately respond to imposed variations in operating conditions, such as a sudden increase in, or a change in the nature of the sewage being treated. The only means for providing such additional oxygen is by increasing the rotational speed of the aerator, but such unavoidably causes an unneccessary and possibly detrimental increase in mixing velocity and therefore tank turbulence. Similarly, attempts to increase aeration rates in turbine aeration and diffused air aeration processes also require or result in an increase in mixing velocity. Thus, in previously known aeration methods, the maximum oxygen input has been limited to that which is accompanied by an acceptable increase in mixing velocity to just below that at which homogenization of the sludge floc will occur.
Apart from the considerations imposed by attendant increase in mixing velocity, previous attempts to increase the amount or rate of absorption of oxygen in the liquor being aerated have not been entirely satisfactory. For example, diffused air aeration techniques involve the introduction of large quantities of compressed air into the mixed liquor via various types of diffusers in a manner not unlike that of an airlift pump. However, these techniques require relatively high power and, because oxygen transfer occurs via the liquid interface of a small rising bubble whose time duration within the liquor is relatively short as it travels to the surface, the oxygen transfer efficiency is comparatively low. Similarly, turbine aeration techniques involve the introduction of such air bubbles beneath a horizontally disposed and rotating circular plate having attached blades for the purpose of shearing the air bubbles to reduced size and dispersing them within the liquor, the turbine blades being at a submerged location near the bottom of the tank where the compressed air is released to provide the air bubbles. Although oxygen transfer efficiency is improved, it is known that turbine aeration involves many of the disadvantages of simple diffused air aeration methods and is therefore not entirely satisfactory.
Other known methods of improving oxygen absorption include the direct introduction of gaseous oxygen, instead of air, to the sewage being treated, the manner of introduction being essentially the same as that involved in diffused air aeration, turbine aeration, and surface aeration processes in both open and covered tanks. However, it is found that much of the introduced oxygen is lost to the surrounding atmosphere, or must be released undissolved from the pressurized environment within which aeration takes place, such that the operating costs of such gaseous oxygen treatment techniques are relatively high.
Of course, any increase in either the amount or rate of dissolving of oxygen within the sewage permits a concommitant increase in the concentration of mixed liquor suspended solids and consequent decrease in the required time of treatment. Such reduction in aeration tank detention time permits reduction of the size of the aeration tank, or enlarged capacity of the system, and other attendant benefits.
As described, for example in U.S. Pat. No. 3,547,815 (McWhirter), it is known that the amount of oxygen ultimately absorbed by the treated liquor (i.e., oxygen utilization) can be improved by reutilizing the pure oxygen or oxygen-containing gas delivered to the first of a series-arranged plurality of oxygenation stages, in each of the succeeding stages. The stages are provided by a number of oxygenation chambers arranged within a common aeration tank, and dissolving of oxygen is promoted within each chamber using any known surface aeration, turbine aeration, or radial brush mixing technique. However, although as high as 90% oxygen utilization is claimed for the system, its power requirements appear to be relatively high, and whichever aeration technique is used will necessarily affect velocities and uniformity of mixing throughout the tank. As previously noted, such is a disadvantage of all such known aeration devices.
In addition to the difficulty of promoting a relatively high percentage of oxygen utilization by the liquor being treated, solving of the separate problem of providing adequate and uniform mixing throughout the aeration tank has been impeded by the aeration process employed. For example, and considering that minimum velocities of about 2.0 feet per second must be maintained in the far corners as well as throughout the aeration tank, the relatively poor pumping efficiency of a submerged turbine or a surface aerator requires what is believed to be excessive energy to achieve the desired velocity at all locations, and excessive velocities are more often attained in the immediate vicinity of the mixing or aerating device. Moreover, such difficulties in establishing uniform and relatively low mixing velocities throughout the aeration tank have imposed natural limitations upon the size or depth of the aeration tank, depending upon the manner in which the aerating process is conducted. That is, the length, width, maximum depth, and shape of an aeration tank has been virtually dictated not only by the aeration technique which is adopted, but also by the required uniformity of low rate mixing throughout the tank.
Improvements in the manner of mixing have resulted in permissibly larger or deeper tanks, but, as previously noted, adoption of such improved mixing techniques has been inhibited by the manner of introduction or use of process air or oxygen.
For example, an improved mixing apparatus which has thus far been used only in water treatment flocculation processes but which has been believed not to be adaptable to sewage and similar biological oxidation processes is a vertically reciprocating paddle-type flocculation unit manufactured by the Ralph B. Carter Co. of Hackensack, New Jersey. It consists of one or more tiers of horizontally disposed planks or beams which are submerged in the aeration tank and moved slowly and reciprocally in vertical direction by a rocker arm mounted above the tank and from which the beams are suspended. In a rectangular tank, the upwardly and downwardly moving horizontal planks extend to the corner areas of the tank as well as within its central regions, and therefore produce very uniform mixing at the desired velocity as determined by the accurately controlled rate of oscillation of the rocker arm.